Page:Text-book of Electrochemistry.djvu/195

 i8o THE DISSOCIATION THEORY. chap.

classed amongst those in which the " retention of the type " takes place against the ordinary chemical rules. By the action of other salts, where exchange of ions takes place, a ferrocyanide always gives a ferrocyanide, and not a cyanide and a ferrous salt. In order to bring about such a rearrange- ment, more energetic chemical means must be employed ; in this case, for instance, potassium ferrocyanide may be heated (fused), and it is decomposed into potassium cyanide, nitrogen, and iron carbide, and this last compound, on treatment with hydrochloric acid, gives ferrous chloride.

As a rule the ions are much more reactive than other chemical substances. The exchange of ions in precipitation and similar reactions takes place, as far as we can judge, instantaneously, whilst other reactions often take place very slowly and only at high temperature with a measurable velocity. We might even go so far as to say that only ions can react chemically. However, it would be difficult to definitely prove this ; but we may assume that in the case of reactions which take place very slowly ions are present, although not in measurable quantity so far as our present methods are able to detect them.

Gore {13) has shown that absolutely anhydrous hydro- chloric acid, which does not conduct an electric current, does not react with the oxides or carbonates of magnesium and the alkaline earth metals, whilst in aqueous solution these substances are violently attacked. Concentrated sulphuric acid does not act upon iron until water is added.

Physiological Action of the Ions. — The physiological actions of different salt solutions as curative agents or poisons are of great practical interest. It has been known for a long time that morphine given in the form of sulphate, chloride, acetate, etc., always has the same effect when used in equivalent quantities. The negative ion present \Nith the morphine has no physiological influence. Similar observations have been made with other substances, such as quinine, etc.

Mention mav here be made of some observations with

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